This invention relates in general to the field of imaging systems and more particularly to an improved method and apparatus for identifying an object and determining its location.
Object detection and ranging has traditionally been done by RADAR(Radio Detecting And Ranging) systems. Radar waves have a relatively long wavelength which creates a broad radar beam. In many detecting situations, the beam covers the entire target so it is difficult to obtain target shape information. Thus, RADAR cannot accurately determine the shape of an object for use in applications requiring target detection, recognition and identification. In recent years, LADAR (Laser Detecting And Ranging) systems have been introduced. LADAR systems use laser beams, which have a short wavelength, in place of the Radio waves. The short wavelength laser beam allows a much narrower beam with which to illuminate the target. The beam may be less than a meter in diameter. Therefore, the narrow laser beam may separately illuminate many points on the target and determine the range to each illuminated area. From these many measurements of range, the three dimensional shape of the target may be obtained allowing the target to be distinguished from non-targets (referred to as xe2x80x9cclutterxe2x80x9d).
A LADAR system is an electro-optical system using a laser as an illuminator and a receiver which detects a return of the laser and converts the time taken for the return to a range value. Currently LADAR systems are of the scanning variety and referred to as scanning LADAR systems In this type of LADAR, a laser is pulsed at a high rate with one pulse per pixel (picture element) or portion of the target area to be studied. The laser is scanned such that each pixel in the field of view (FOV) is illuminated and detected. Each laser pulse may be reflected from the target area and received by a detector at the LADAR site. The receiver has a collection aperture and a detector with an instantaneous field of view (IFOV) which corresponds to the portion of the target area which is illuminated by the laser beam. The receiver IFOV is less than the FOV and hence defines the pixel location within the FOV.
The detector and the laser are scanned congruently so that the detector is always positioned to receive any reflected laser beams from the most recent laser pulse. After the entire target area has been scanned, the LADAR system has enough information to determine the approximate range to any object within the target area and the approximate shape of any detected object. Since the LADAR system can be mobile and objects within the target area are often mobile, distortion is introduced into the scanning LADAR system model due to the relative movement of the laser source and the target object during the scan of the LADAR. These distortions are manifested in inaccurate range determinations and imprecise shapes. In addition, purely analog range processors, such as those used with current scanning LADARs, are not as accurate as all digital or hybrid analog-digital range processors.
Flash LADAR systems have been introduced which solve the problems associated with scanning LADAR systems. However, flash LADAR systems have heretofore only been built on a demonstration basis and have been impractical due to their excessive size. Flash LADAR systems illuminate the entire target area with a single pulse from the laser. This laser pulse is then reflected from any object within the target area. Next, the reflected laser pulse is received and the object detected. A flash LADAR system requires many detectors arranged in a grid pattern, referred to as a detector array, so that it can recreate the target area and effectively determine the range and approximate shape of any objects contained within the FOV. The range and shape information may be used to identify an object and to determine the location of the object. Current detectors and their associated signal processing circuitry are constructed of analog circuitry which prevents the practical construction of a detector array since it would be excessively large A detector array followed by a fully digital processor would be optimal in terms of size. However, the data received by the detector array would be in excess of the capacity of any known processing device. Therefore, an all digital implementation of a detector array is not possible.
Accordingly, a need has arisen for a method and device in the area of detection systems which increases the accuracy of range, location, shape, and identity determinations and eliminates the distortions and inaccuracies associated with scanning LADAR systems. In accordance with the present invention, a device and method for determining a range and shape of an object are provided which substantially eliminate or reduce disadvantages and problems associated with conventional detection systems.
According to one embodiment of the present invention, a detection device is provided that comprises a detector which will receive laser light pulses which are reflected from an object in a target area. A data sampler then takes samples of the amplitude of the reflected laser light pulse. The detection device then compares the amplitude of each data sample to a predetermined threshold value. Once a data sample exceeds a threshold value, a computer using interpolation techniques can identify the object and determine a location of the object.
One important technical advantage of the present invention arises from the fact that inaccurate range and shape determinations in scanning LADAR systems caused by the relative movement of the laser source and the target are eliminated. The relative target and LADAR motion causes distortion since the scanning of the target takes time, such as one half second, and hence the range to various pixels on the target is measured at different times and distortions in the target shape occur. In contrast, the ranges in a flash LADAR are measured simultaneously in a single pulse, and therefore relative motion distortion does not occur. The flash LADAR effectively measures the range to each pixel and hence the target shape at the same instant in time The present invention allows flash LADAR systems of a practical size to be constructed. If a purely analog range processor, such as used with scanning LADARs, is connected to each detector, the volume of the processor would be prohibitively large. A purely digital processor may well have some volume advantages, but today""s analog to digital converters (ADC) are far too slow to meet the processor requirements. The present invention uses a hybrid analog/digital processor which is small and compact enough to allow a flash LADAR of practical size to be constructed and yet provides highly accurate range measurement for target acquisition.
Yet another important advantage of the present invention relative to scanning LADAR systems is mechanical simplicity. The scanning LADAR requires complex and bulky mechanical or electronic scanners to scan the transmitted and returned laser beams. The flash LADAR does not require a scanner. In addition, the scanning LADAR must be stabilized to preserve the registration between pixels. Mechanical motion and jitter do not affect pixel registration in a flash LADAR since all pixel range measurements are made simultaneously during the instant the flash illuminates the target.
Another important technical advantage of the present invention arises from the fact that more accurate range and shape determinations are made using a hybrid analog/digital range processor rather than an all analog range processor. The hybrid analog/digital range processor may be used with scanning LADAR systems to provide more accurate range and shape determinations.